The design of skate wheels has changed considerably as skates have evolved from traditional roller skates, with four wheels arranged in parallel sets of two wheels each, to in-line skates. The in-line skates require rugged, "high performance" wheels, which are narrower than the wheels used in traditional roller skates. These narrower wheels allow skaters to perform, for example, the aerial stunts often associated with in-line skates.
The skate wheels currently used on in-line skates consist of a molded plastic center hub and a hard plastic, typically, urethane, outer section, which is molded directly onto the center hub. The urethane outer section is shaped, or graded, such that the portion that is in contact with the road is relatively narrow in comparison with the portion that is molded to the center hub. These wheels give the skate a fast, smooth ride when they are new. As the wheels wear, however, the narrower portion gives way to the wider portion and more wheel surface is in contact with the road. This results in increased friction, which slows the skates. Also, the wheels tend to wear unevenly because of the side-to-side motion of the skates, which adversely affects the performance of the skate.
Once the wheels wear down to the point where they noticeably and adversely affect the performance of the skate, they must be replaced. The user discards the four wheels and attaches four new wheels. These wheels are relatively expensive, both because of the materials used to make the wheels and because of the two-step molding process, that is, the molding of the center hub and then the molding of the urethane outer section onto the hub.